Three parts together must preserve a clearance gap. How do you stop the tolerance stack from eating that gap?
Start at the functional dimension
First determine what needs to fit, move or seal. Choose reference surfaces that match the function and the measurement method. Avoid a chain of dimensions when one shared reference can capture the function directly. A CAD assembly that closes up nominally doesn't prove that every produced part will fit.
A simple worst-case stack
Suppose an opening is 30 ±0.10 mm and a part is 29.5 ±0.15 mm. The nominal clearance is 0.50 mm. The smallest clearance is 29.90 − 29.65 = 0.25 mm; the largest is 30.10 − 29.35 = 0.75 mm. If you add shims, coatings or extra parts, include their contributions explicitly in the chain.
Make assembly and maintenance accessible
Provide lead-in chamfers, room for tools and a workable assembly sequence. Determine which parts locate the assembly and which only clamp it. Check whether a wear part can be replaced without realigning the entire machine. An exploded view helps make this sequence easy to discuss.
Assumptions and applicability limits
The example uses a linear worst-case sum of dimension limits. Form, position, temperature and deformation are not included. Don't choose a fit from a general table alone: load, material, assembly and manufacturer data determine what's appropriate. No standard conformance is claimed here.